October 2005

5

MIC2198

MIC2198

Micrel, Inc.

Block Diagram

Current

Limit

Error

CORRECTIVE

RAMP

RESET

Reference

Oscillator

EN/UVLO

2

7

10

6

12

11

8

9

4

5

3

1

COMP

100k

MIC2198

FB

R2

R1

VOUT

CSH

VDD

BST

VIN

HSD

Q2

Q1

D1

D2

L1

VSW

LSD

PGND

4.7µF

V

0.8V

R1

R2

+

1

V

6 V

Current

Control

PWM









Logic

Amp

Sense

Amp

Figure 1. Internal Block Diagram

Functional Description

The MIC2198 is a BiCMOS, switched-mode, synchronous

step-down (buck) converter controller. Current-mode control

is used to achieve superior transient line and load regulation.

An internal corrective ramp provides slope compensation

for stable operation above a 50% duty cycle. The controller

is optimized for high-efficiency, high-performance DC-DC

converter applications.

The MIC2198 block diagram is shown above.

The MIC2198 controller is divided into 5 functions.

• Control loop

• Current limit

• Reference, enable and UVLO

• MOSFET gate drive

• Oscillator

Control Loop

The MIC2198 operates in PWM (pulse-width-modulation)

mode. In PWM mode, the synchronous buck converter

forces continuous current to flow in the inductor which also

improves cross regulation of transformer coupled, multiple

output configurations.

PWM Control Loop

TheMIC2198usescurrent-modecontroltoregulatetheoutput

voltage. This method senses the output voltage (outer loop)

and the inductor current (inner loop). It uses inductor current

and output voltage to determine the duty cycle of the buck

converter.Samplingtheinductorcurrentremovestheinductor

from the control loop, which simplifies compensation.

A block diagram of the MIC2198 PWM current-mode control

loop is shown in Figure 2 and the PWM mode voltage and

current waveform is shown in Figure 3. The inductor current